diff --git a/include/olm/inbound_group_session.h b/include/olm/inbound_group_session.h
index ef01038..9b2f9df 100644
--- a/include/olm/inbound_group_session.h
+++ b/include/olm/inbound_group_session.h
@@ -25,7 +25,7 @@ extern "C" {
 typedef struct OlmInboundGroupSession OlmInboundGroupSession;
 
 /** get the size of an inbound group session, in bytes. */
-size_t olm_inbound_group_session_size();
+size_t olm_inbound_group_session_size(void);
 
 /**
  * Initialise an inbound group session object using the supplied memory
diff --git a/include/olm/olm.h b/include/olm/olm.h
index 5764eb2..838d817 100644
--- a/include/olm/olm.h
+++ b/include/olm/olm.h
@@ -39,13 +39,13 @@ typedef struct OlmUtility OlmUtility;
 void olm_get_library_version(uint8_t *major, uint8_t *minor, uint8_t *patch);
 
 /** The size of an account object in bytes */
-size_t olm_account_size();
+size_t olm_account_size(void);
 
 /** The size of a session object in bytes */
-size_t olm_session_size();
+size_t olm_session_size(void);
 
 /** The size of a utility object in bytes */
-size_t olm_utility_size();
+size_t olm_utility_size(void);
 
 /** Initialise an account object using the supplied memory
  *  The supplied memory must be at least olm_account_size() bytes */
@@ -66,7 +66,7 @@ OlmUtility * olm_utility(
 );
 
 /** The value that olm will return from a function if there was an error */
-size_t olm_error();
+size_t olm_error(void);
 
 /** A null terminated string describing the most recent error to happen to an
  * account */
diff --git a/include/olm/outbound_group_session.h b/include/olm/outbound_group_session.h
index 663f1d2..eafaf80 100644
--- a/include/olm/outbound_group_session.h
+++ b/include/olm/outbound_group_session.h
@@ -25,7 +25,7 @@ extern "C" {
 typedef struct OlmOutboundGroupSession OlmOutboundGroupSession;
 
 /** get the size of an outbound group session, in bytes. */
-size_t olm_outbound_group_session_size();
+size_t olm_outbound_group_session_size(void);
 
 /**
  * Initialise an outbound group session object using the supplied memory
diff --git a/src/inbound_group_session.c b/src/inbound_group_session.c
index a78404d..7e96294 100644
--- a/src/inbound_group_session.c
+++ b/src/inbound_group_session.c
@@ -58,7 +58,7 @@ struct OlmInboundGroupSession {
     enum OlmErrorCode last_error;
 };
 
-size_t olm_inbound_group_session_size() {
+size_t olm_inbound_group_session_size(void) {
     return sizeof(OlmInboundGroupSession);
 }
 
diff --git a/src/olm.cpp b/src/olm.cpp
index d3af19c..d626c84 100644
--- a/src/olm.cpp
+++ b/src/olm.cpp
@@ -104,7 +104,7 @@ void olm_get_library_version(uint8_t *major, uint8_t *minor, uint8_t *patch) {
     if (patch != NULL) *patch = OLMLIB_VERSION_PATCH;
 }
 
-size_t olm_error() {
+size_t olm_error(void) {
     return std::size_t(-1);
 }
 
@@ -131,16 +131,16 @@ const char * olm_utility_last_error(
     return _olm_error_to_string(error);
 }
 
-size_t olm_account_size() {
+size_t olm_account_size(void) {
     return sizeof(olm::Account);
 }
 
 
-size_t olm_session_size() {
+size_t olm_session_size(void) {
     return sizeof(olm::Session);
 }
 
-size_t olm_utility_size() {
+size_t olm_utility_size(void) {
     return sizeof(olm::Utility);
 }
 
diff --git a/src/outbound_group_session.c b/src/outbound_group_session.c
index 2019844..74ce68d 100644
--- a/src/outbound_group_session.c
+++ b/src/outbound_group_session.c
@@ -43,7 +43,7 @@ struct OlmOutboundGroupSession {
 };
 
 
-size_t olm_outbound_group_session_size() {
+size_t olm_outbound_group_session_size(void) {
     return sizeof(OlmOutboundGroupSession);
 }